The present invention relates generally to a welding system, and more specifically to a method and an apparatus for predicting the life expectancy of a water cooled power cable used in a welding system.
A welding system is used to fuse two or more work pieces together. A welding system accomplishes this by creating a weld seam between the two work pieces. The weld seam is formed by placing a molten metal between the two work pieces to create a bond which holds them together. The molten metal may be created by passing a strand of weld wire through a weld torch and then applying a large amount of current to the weld torch so as to create an electrical arc thereby generating the heat necessary to melt the weld wire and form the molten metal.
In many automated welding systems, robots, known as welding robots, are used to create the necessary welds. Welding robots are controlled by software being executed on a computer which is connected to the welding robot by interface cables. A welding robot has a mechanical arm for holding the weld torch. This arm is generally designed with a number of flexible joints to allow the mechanical arm to move in a variety of directions in order to reach the various weld locations on the work piece.
A power cable is connected between the weld torch and a power source. The power cable must therefore be capable of flexing to accommodate movement of the mechanical arm of the welding robot. Additionally, the power cable must be capable of withstanding extremely high temperatures associated with high power requirements placed upon it. Hence, many modern weld power cables are water cooled. A water cooled power cable consists of a number of copper or other highly conductive metal strands intertwined to create a cable which is surrounded by a liquid impermeable sleeve. Water is pumped through the sleeve. The water removes the heat from the copper cable. This type of water cooled power cable generally has a longer useful life than do power cables which are not water cooled.
Over the life of the water cooled power cable, the copper strands of the cable tend to break as a result of the repeated twisting and bending of the mechanical arm of the welding robot. As this occurs, the electrical current carrying capability of the water cooled power cable is reduced. If the current of the welding system is not maintained at a certain level, the quality of the welds produced by the welding robot will degrade. Eventually, the resistance of the cable will become so great due to the breakage of additional copper strands, that the cable will ignite, incinerate and thereby destruct. Often, other components of the welding robot are damaged or destroyed as the cable incinerates. This results in costly repairs to the welding robot along with extended downtime.
Welding systems have heretofore been designed which include an electrical current detector which measures the magnitude of a specific current in the welding system. If the current falls below a predetermined level, an audible alarm is given to the operator of the welding system so that the operator can take the necessary steps to correct the problem. However, this type of monitoring system does not reveal the cause of the low current level.
Further, U.S. Pat. No. 5,262,609 issued to Nowak et al teaches a self-diagnosing resistance welding cable and method for monitoring the relative increase in cable resistance while the cable is in use, without disconnecting the cable. The cable incorporates a reference resistor in one of the terminals in thermal communication with the cable, such that a comparison in resistance variation between the cable and reference resistor cancels the effects of temperature variation on resistance change. The reference resistor is connected in series with the cable in an electrical test circuit. A reference voltage is applied across the test circuit each time the welding tips of the robot are opened.
Some of the aforementioned designs require that the welding operation be interrupted so that a determination can be made as to whether the water cooled power cable has electrically deteriorated sufficiently as to require replacement. This interruption in the welding operation causes a reduction in performance of the welding system. In particular, the number of welds the welding system can create over a given period of time is reduced. Moreover, the aforementioned designs do not provide any feedback control to the welding process, but rather only predict when the water cooled power cable is at the end of its useful life.
What is needed therefore is an apparatus and method that measures the electrical properties of a water cooled power cable to predict the life expectancy of the power cable which does not reduce the welding performance of a welding system. Moreover, it is desirable to provide an apparatus and method which provides feedback data to the welding system in order to reduce the welding defects resulting from the wear of the water cooled power cable.